As buildings, municipal piping systems, and industrial facilities continue to grow in both size and capacity, the need to verify that the components used to build these systems are working properly and free from defects will continue to rise. Due to the nature of the structural components, it is simply not possible to test each and every element prior to its use. Thus, a need exists for a way to test the components in these structures without destroying the structure during the testing. Non-destructive testing allows for one to test these structures and components while they are being used or after they are already part of a structure without harm to the component or structure. The term “non-destructive testing” (NDT) refers to a diverse group of analysis techniques used in the sciences and in industry to evaluate the properties of a material, component or system without causing damage to the material, component or system as a result of the testing. The value of such testing methods is easily appreciated, because the item being analyzed could be expensive or already part of a larger structure. Thus, non-destructive testing is a highly valuable technique that can save both money and time in product evaluation, troubleshooting, and research.
Because non-destructive testing methods are advantageous, many different techniques for conducting non-destructive testing have been developed. Some of the more common methods include ultrasonic, magnetic-particle, liquid penetrant, radiographic, and eddy-current testing. These various methods provide a means for evaluating materials that is useful to many industries including forensic engineering, mechanical engineering, electrical engineering, civil engineering, systems engineering, medicine, nuclear regulation, and others.
Of these various methods of non-destructive testing, perhaps one of the most widely used is ultrasonic testing. In ultrasonic testing, very short ultrasonic pulse-waves with center frequencies ranging from 0.1-15 MHz and occasionally up to 50 MHz are launched into materials to detect internal flaws or to characterize materials. Ultrasonic testing is also commonly used to determine the thickness of the test object, particularly when it would be impossible to determine the thickness of a material without breaking it open. An example of this is the testing of pipes. Ultrasonic testing is often used to measure the thickness of pipes that are part of a piping system and may have suffered from corrosion. Without a non-destructive testing method, the pipe would have to be removed from the pipe system and checked for corrosion. Obviously this would be time consuming, expensive, and potentially lead to further problems with removing and replacing certain sections of pipe. Use of a non-destructive technique such as ultrasonic testing allows one to evaluate and analyze the pipe without removing it from the system or breaking it open. The ultrasonic testing can determine the pipe thickness and this information can disclose whether the pipe is suffering corrosion and, indeed, to what extent it has already corroded.
Ultrasonic testing is basically a way to pass sound waves through a structure and measure the waves reflection or diffraction as they pass through the material comprising the structure. In ultrasonic testing, an ultrasound transducer connected to a diagnostic machine is passed over the object being inspected. The transducer, which is responsible for sending out the sound waves, is typically separated from the test object by a thin barrier called a “couplant.” Typically the couplant of choice is gel, oil, or water, as in immersion testing.
Ultrasonic testing usually involves one of two methods of receiving the ultrasound waveform, reflection or diffraction. In a reflection configuration (typically referred to as “pulse-echo”) the transducer performs both the sending and the receiving of the ultrasonic sound waves as the sound or wave is reflected back to the transducer from which it originated. As the ultrasonic sound waves pass through the material or object, any change in the consistency of the material or object will cause a reflection of the wave. Reflected ultrasound, therefore, results whenever a wave comes into contact with an imperfection in the material. In addition, like an imperfection within the material, the “back wall” (back surface) of the material will also cause a reflection of the ultrasonic waves. This is due to the transition from the material to either air or whatever material is on the other side of the material being analyzed. Thus, the back wall of a pipe will cause a reflection of the ultrasonic sound waves. The transducer that sent out the ultrasonic sound waves is configured to receive the reflected waves from the broadcasted ultrasonic waves. In essence, the transducer listens for the echo from the reflected ultrasonic waves
In contrast to a pulse echo setup, ultrasonic testing also can employ a through-transmission setup. In a through-transmission setup, the transducer broadcasts an ultrasonic wave through the material being tested, and a separate receiver on the other side of the material detects the signal. The separate receiver listens for the ultrasonic waves and records the waves as they complete passage through the material being tested. Like the pulse-echo setup, the amount and timing of ultrasonic waves that reach the receiver after traveling through the material will indicate severity and location of imperfections or other conditions in the material being tested. Any space or imperfections between the transmitter and receiver will reduce the amount of sound waves that pass through, thus revealing the presence of the imperfection or transition.
Typically, the transducer is coupled with a monitoring system or printer such that the instrument can display the results in the form of a signal with a distinct amplitude. The amplitude of the signal, represented by peak height, represents the intensity of the reflected sound wave. The distance between the peaks is typically proportional to and represents the distance between the surface of the material being analyzed and the imperfection, flaw, back wall, geometry, or other obstruction that caused the reflection of the ultrasonic wave. This distance is directly proportional to the arrival time of the reflection.
Of particular interest to the instant application is the method of ultrasonic non-destructive testing using a phased array. In a phased array, multiple transducers are used to generate a plurality of ultrasonic pulses. These ultrasonic pulses can be steered by varying the time delay at which the ultrasonic wave is pulsed. These delays are applied during emission and reception of the ultrasonic signals. By varying the time the waves are pulsed, the resultant wave front can be steered. This results in the ability to focus the beam and scan a larger area from a fixed position due to the ability to sweep the beam by varying the time delay in the phased array.
Unlike a phased array, a conventional probe is able to obtain a fixed beam angle by using an angled wedge. The obtained angle of refraction is defined by the wedge angle and the propagation speed in the wedge and in the inspected material (Snell's law). Due to the time delay introduced by the wedge, the time of emission (at the interface) is different. The waves generated from the shortest wedge path will start to propagate into the material sooner, while these generated from the longest wedge path will start entering the material later. At a specific point in the material, all the waves will be in phase (focused) and will continue propagating into the material while creating a wave front. The wave front travels into the material at a given angle (angle of refraction) following the delays that occur at the interface level. If the wedge angle or the propagation velocity of the wedge is changed, the delays are different and consequently a beam with another angle of refraction will be generated. Thus, one can steer the beam and control where the beam is focused.
Use of ultrasonic non-destructive testing to analyze materials is well known in the prior art. Ultrasonic transducers which have been used in pulse-echo mode to locate flaws and defects in tubular goods are well known. In the pulse-echo mode, the ultrasonic transducer emits an ultrasonic wave and then waits to receive an echo from the defect. The angle of incidence and angle of reflection relative to the surface of the defect must be approximately equal. As a result, a transmitting transducer can only receive an echo from the defect surface which is approximately normal to the direction of ultrasonic wave transmission. If the defect surface is more than about five degrees off-normal to the direction of propagation, the ultrasonic wave will be reflected but will not return optimal acoustic energy to the transmitting transducer for the defect to be detected.
Ultrasonic transducers have also been used in the past in pulse-echo mode to generate ultrasonic shear waves which travel circumferentially around the tubular goods being examined, and to detect axially-oriented flaws. Axially oriented ultrasonic transducers have also been used to generate axial shear waves and to detect circumferentially-oriented flaws. For some purposes, ultrasonic transducers have been oriented perpendicular to the examined surface, for instance to determine wall thickness, and have been operated in a pulse-echo mode.
Utilizing separate transmitting and receiving transducers is commonly referred to as a pitch-catch configuration. Each transducer commonly comprises a piezoelectric element and is mounted in a block of suitable material to form a search unit. Upon receipt of an electronic signal, each piezoelectric element transmits an ultrasonic signal into the material with which the search unit is in acoustical contact. Conversely, upon receipt of a suitable ultrasonic signal from the material, each piezoelectric element produces an electric signal proportional to the pressure amplitude of the ultrasonic signal incident on the element. The amplitude and shape of the voltage signal produced upon receipt of an ultrasonic signal reflected from a particular flaw or other abnormality provides information about the flaw or abnormality. Thus, flaws, abnormalities, or other deviations from the material are detected and can be measured. Prior art uses of ultrasonic inspection have utilized the receiving transducer to receive sound by placing the transmitting and receiving elements generally facing one another and measuring signal loss due to the blockage of part of the sound wave before being received by the receiving transducer.
The primary advantage of a pitch-catch configuration versus a pulse-echo configuration is found when analyzing material with a low velocity that tends to be very attenuative. When using ultrasonics to test these types of materials, extra gain in the ultrasonic signal must be applied to adequately penetrate the low velocity, highly attenuative material. Unfortunately, problems can occur when the gain is raised to higher levels. A problem with the higher gain comes in the form of increased wedge and material noise. Thus, if a pulse-echo configuration is used as opposed to a pitch-catch configuration, the electronic probe can receive noise caused by reflection of the ultrasonic waves from the wedge itself or other variables not present in a pitch-catch configuration. Thus, in order to reduce the noise level, a pitch-catch configuration is utilized because it only receives the ultrasonic responses from the material and not from the wedge itself.
In any automatic ultrasonic flaw detection system, a major capital cost outlay is the area of the system's electronics for the flaw detection signal processing. The number of channels of a particular system will determine the number of transducer probes from which it can process signals. In order for an automated system to operate at high speeds, a wide scanning width is needed. This can be achieved by using large area single element probes for each channel of electronics, but the decrease in resolution of these larger probes often mandates smaller probes with a decreased inspection scan width. As a result, this increases the number of channels that a system will require for a given scan width.
In the past, various patents have issued relating to ultrasonic inspection devices and techniques. For example, U.S. Pat. No. 4,305,297, issued on Dec. 15, 1981, to Ries et al. teaches ultrasonic test equipment for testing the welding seam on a thick wall. This includes transducers arranged in tandem and along the welding seam. In this invention, a tandem pair of transducers scans for defects or abnormalities in the material. In addition, a third transducer is configured to capture data related to flaws and abnormalities in the material.
U.S. Pat. No. 4,522,064, issued on Jun. 11, 1985, to J. D. McMillan provides an ultrasonic method and apparatus for determining the depth of cracks in a pipe or conduit. This apparatus includes a transmitting transducer and a receiving transducer which are placed on the outside surface of the pipe. The transmitting transducer is energized to direct a shear wave beam of ultrasonic energy at the crack so as to generate a complex reflected wave front from the crack. This wave front contains the information as to the size of the crack in relation to the wall thickness. The receiving transducer is moved relative to the transmitting transducer until a peak or maximum amplitude reading is found.
U.S. Pat. No. 4,523,468, issued on Jun. 18, 1985, to Derkacs et al. provides a phased array inspection of cylindrical objects. A first array of ultrasonic transducers transmits ultrasonic shear waves circumferentially around an examined cylindrical object. A second array transmits ultrasonic shear waves axially along the examined object. Triggering pulses from a triggering amplifier are switched by a multiplexer to each individual transducer of the first and second arrays. As one of the transducers assumes the role of a transmitting transducer and transmits an ultrasonic wave, the other transducers of the first and second arrays assume a receiving mode to receive reflected ultrasonic components, i.e. a pitch-catch setup.
U.S. Pat. No. 4,641,531, issued on Feb. 10, 1987, to Reeves et al. describes an ultrasonic inspection apparatus for locating multiple defects in wall tubing. A plurality of transducers are arranged in mated pairs, each of the pairs including a sender element for transmitting an ultrasonic shear wave and a receiver element for receiving a reflected ultrasonic wave component from the tubular goods being inspected. Each sender element is a point focus transducer having sufficiently high resolution to maintain detectability of defects in the tubular goods.
U.S. Pat. No. 5,165,280, issued on Nov. 24, 1992, to Sternberg et al. provides a device for testing oblong objects through the use of ultrasonic waves. A transmitting ultrasonic transducer generates ultrasonic waves that are acoustically irradiated into the respective object via a coupling medium. At least three ultrasonic transducers are arranged in a row next to one another along the line. The transducers are aligned in an array wherein the outer transducers are set to receive the ultrasonic signals from the central transducer. The line array is inclined along the longitudinal axis of the object by an angle which is between zero degrees and forty-five degrees.
U.S. Pat. No. 5,189,915, issued on Mar. 2, 1993, to Reinhart et al. shows a single mode ultrasonic inspection method and apparatus. This is adapted to utilize both pitch/catch and pulse/echo information obtained by using a single mode of wave propagation. The apparatus includes an ultrasonic signal transmitting assembly, a signal receiving assembly, a positioning mechanism, and an arrangement for recording the information received by the signal receiving assembly. The transmitting assembly includes at least one source transducer for transmitting shear mode ultrasonic search signals into the mass of the object to be inspected. The signal receiving assembly receives shear mode ultrasonic signals that are produced as the ultrasonic search signals encounter discontinuities in the mass of the object being inspected. The signal receiving assembly also receives shear mode ultrasonic echo signals that are directed back to a first source transducer.
U.S. Pat. No. 5,431,054, issued on Jul. 11, 1995, to Reeves et al. discloses an ultrasonic flaw detection device including a transmitting ultrasonic transducer for producing multiple shear wave skips between inner and outer surfaces of a test object. It includes a plurality of receiving transducers positioned ahead of and parallel to at generally the same incident angle as the transmitting ultrasonic transducer. The device also includes a processor. The receiving transducers serve to receive energy of the shear wave as reflected from discontinuities or flaws in the object. Each of the receiving transducers is electronically isolated from the other receiving transducers. The receiving transducers are connected to a single channel of the processor. A fixture is provided having a first opening for detachably receiving the transmitting ultrasonic transducer and a plurality of receiving openers receiving the receiving transducers therein.
U.S. Pat. No. 6,736,011, issued on May 18, 2004, to Zayicek et al. discloses a method of ultrasonically inspecting an area around a disk bore and a keyway on a shrunk-on steam turbine disk that is attached to a rotor. The method includes placing one or more phased array ultrasonic probes on at least one face of the disk across from the keyway area without disassembling the disk from the rotor. The array then transmits ultrasonic energy in the form of one or more beams from one or more ultrasonic probes to scan along the disk bore and keyway area. Thus, flaws are detected and the waves that are reflected and diffracted from the flaws in the disk bore and keyway area are used to simultaneously reconstruct an image of the disk bore and keyway area. Thus, the image is used to detect and locate defects within the disk bore and keyway area.
U.S. Pat. No. 7,328,619, issued Feb. 12, 2008, to Moles et al. discloses an ultrasonic probe suited for testing the integrity of sheet metal surfaces around fastener openings. The probe uses means to center the probe over the opening including a mechanical centering rod and an electronic display. Once positioned within tolerance limits, phased array ultrasonic beams search for defects within the metal surfaces, allowing for residual offsets in the centering of the probe.
U.S. Pat. No. 7,428,842, issued Sep. 30, 2008, to Fair et al., discloses a phased array ultrasonic testing system for examining turbine disc bores and blade attachments for discontinuities, such as stress corrosion cracking. The system includes a control system with a computer and a controller for programming, emitting, and steering an ultrasonic beam via at least one two-dimensional phased array probe, thereby precisely inspecting the area of interest while simultaneously accommodating complex geometry of the disc or blade attachment. Computer control of the beam permits the number of inspection locations and the number of different probe wedges to be reduced.
U.S. Pat. No. 7,624,651, issued on Dec. 1, 2009, to Fernald et al. discloses and teaches an apparatus for damping an undesired component of an ultrasonic signal. The apparatus includes a sensor affixed to a pipe. The sensor includes a transmitter and a receiver. The transmitted ultrasonic signal includes a structural component propagating through the pipe and a fluid component propagating through a flow in the pipe. The receiver receives one of the transmitted components. The apparatus includes a damping structure. The damping structure dampens the structural component of the ultrasonic signal to impede propagation of the structural component to the receiver. The damping structure includes one of a housing secured to the pipe to modify ultrasonic vibrational characteristics thereof, a plurality of film assemblies including a tunable circuit to attenuate structural vibration of the pipe, and a plurality of blocks affixed to the pipe to either reflect or propagates through the blocks, the undesired structural component of the ultrasonic signal
A significant problem in each of these prior art patents is present. The problem, inherent in phased array non-destructive testing, is the inability to scan directly under the surface of the material being scanned with a stationary probe. Because the phased array transducer, or any other non-destructive testing device, must rest on top of the material being scanned, there is an inability to scan within a certain angle from the surface of the material. In order to detect flaws near the surface of the material, the probe must be indexed (moved) in order for the ultrasonic waves to contact and thus detect the near-surface flaw. Near a joint where the probe cannot pass directly over the flaw, the only way a flaw can be detected is with a bounced beam. The ultrasonic beam is sent down into the material and bounced off the other wall back toward the surface upon which the device is resting. This way, the beam can detect the flaw, but only because the beam has already traveled the full thickness of the material and been reflected. Due to the reflection, the beam has become weakened, thus producing a much weaker signal. Thus, there exist places where a flaw can exist but cannot be detected without a reflected beam. Such a flaw is represented in FIG. 9. The three flaws in the material, each represented by a dot, are searched for by the phased array detector. However, as seen in the diagram, the prior art detector is unable to sweep the ultrasonic waves at an angle high enough to detect the topmost flaw, closest to the surface. Because of the nature of the wedge material and angle, the topmost flaw remains undetected by known ultrasonic phased array non-destructive testing. This is a problem because known techniques of non-destructive, phased array testing can miss flaws in materials because of the difficulty in sweeping the beam at such a high angle. Thus, there is a need for an apparatus and method capable of detecting flaws near the surface of the material being tested. Further, there is a need for an apparatus and method of testing that can utilize a phased array and sweep the beam at higher angles than are possible with current technology. The present invention fulfills these needs and solves these problems.
Thus, it is an object of the present invention to overcome the limitations found in the prior art and known apparatuses and methods used for non-destructive testing of materials with phased-arrays. It is a further object of the present invention to provide an ultrasonic detection system that can increase the effective depth coverage without a significant loss of resolution. It is a still further object of the present invention to provide an ultrasonic detection system that can increase the effective angle of sweeping of the ultrasonic waves without increasing the sound path of the ultrasonic waves. It is still a further object of the present invention to provide an ultrasonic inspection system that satisfies the goals and overcomes the problems in the prior art while still remaining easy to manufacture, easy to use, relatively inexpensive, and very accurate. The present invention achieves all of these goals and these and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.